The timing of motherhood in the UK has become increasingly socially polarised over time. Early motherhood often occurs among those who have been raised in disadvantaged circumstances whilst later motherhood is associated with having had better upbringings and having taken advantage of the education system. This paper considers the potential impact on her children of the divergence in the family life marked by the age at which a woman becomes a mother for the first time. Using the UK Millennium Cohort Study, we present evidence on the impact of the timing of a mothers’ first birth on cognitive and behavioural outcomes of the cohort child. These outcomes are measured in the third sweep of the MCS when the cohort members are five years old. We control for some of the life course experience of the cohort child’s mother, and then also circumstances in the child’s first year. We demonstrate that much of the difference observed between the cognitive and behavioural outcomes of the children of young and older mothers is the consequence of either their mothers’s disadvantaged social origins, or disadvantages apparent at the time of the first survey, which may also have had earlier origins. There is only a small degree of developmental penalty left to be attributed to the mother’s age per se.